Q.
From (name withheld) in New York: The Esophus Creek in the Catskills of NY has been
steadily declining as a once pristine water and fishery. We are
presently suing NYC DEP to restore this stream back to its original state.
What I want to know is, what affects does sedimentation and turbidity have
on aquatic life, the spawning of rainbows and browns and the future of a
stream, with this famous stream being now known as the Yoo-Hoo Creek?

A.
A good question, and one that has several facets to it. Let me
address the impacts of sedimentation on the different levels of the
food chain.

Algae/diatoms: These organisms form the base of the food web, or at
least one of the main bases, and is impacted by sedimentation in three
ways. First, by its presence in the water column, suspended sediments
(turbidity) decrease the amount of sunlight penetrating to the bottom
of the stream, thus effectively shading the algae/diatoms and decreasing
their potential for production of oxygen and growth. Second, the
suspended sediments act as an abrasive, effectively scouring algae/diatoms
from the substrate. Third, suspended sediments that settle to the bottom
cover the algae/diatoms present, thus cutting them off from both light
and oxygenated water and resulting in their death. All of these
impacts decrease the amount of algae/diatoms on the bottom, the prime
food resource for grazing/scraping insects.

Invertebrates: Immature insects which utilize the surface of stones
for their food source (grazers/scrapers) or living site (many filterers)
are impacted by sedimentation. The former are impacted as described
above, and the latter are impacted by being smothered from sediments
that settle on them. Fine sediments are extrememly detrimental to
mayflies because they settle on and adhere to their gills, effectively
smothering them. Sediments that settle and fill the spaces between
stones on the bottom of streams effectively eliminate these sites for
habitation by invertebrates which seek out these places for shelter
and food.

Trout: Again, there is a dual impact here. First of all, if the
sediments settle and fill up the spaces between the gravels that
trout prefer for spawning, the fish will simply avoid these places
for digging their redds. Secondly, if sedimentation occurs after
the fish have spawned, it will fill the spaces between the gravels
and prevent the flow of aerated water to developing eggs - smothering
and killing them.

That's what sediments do to organisms in the streams; the future,
obviously, is pretty bleak. ~ Stream Doctor

The 'Stream Doctor' is a retired professional stream ecologist and
author, now living in the West and spending way too much time
fly-fishing. You are invited to submit questions relating to
anything stream related directly to him for use in this Q & A Feature
at streamdoctor@aol.com.

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